The invention relates generally to apparatus for tensioning the needle thread of a sewing machine. More particularly, the invention is directed to a sewing machine thread tension control system comprising an infinitely variable thread tension control device and a tortuous path thread tension control device.
In general, prior art needle thread tensioning devices for sewing machines can be divided into two broad categories. Tortuous path type tension control devices and infinitely variable type thread tension control devices. Tension control devices of the tortuous path type include apertured rods and various shaped posts through which or around which the thread that is to be tensioned is threaded or wrapped. This type of tension control device was probably the earliest type of tension control device employed to control the dispensing of needle thread on a sewing machine. Problems with this type of thread tension control device stem from the fact that the tension provided can only be varied in a stepwise or quantum manner and thus cannot be easily varied for different sewing conditions. Many of these prior art thread tension control devices also require somewhat of a complicated threading procedure.
This led to the development of infinitely variable thread tension control devices, the most common of which is known in the art as a tension disc assembly. The conventional tension disc assembly includes a pair of cooperating thread tension discs journaled on a common shaft. The thread to be tensioned is threaded between the tension discs. The contact pressure and thus the thread tension provided by the tension discs is determined by tension springs which exert a pressure on one or both of the tension discs. The contact pressure exerted by the discs is infinitely adjustable with the aid of a tension regulating nut.
The tension control system most commonly supplied on sewing machines today includes a tension disc assembly in combination with a take-up arm and check spring. Needle thread is threaded from the spool around the tension disc assembly through the check spring, the take-up arm and the needle. In operation, the needle moves downwardly penetrating the fabric and dragging a loop of thread through the fabric. As the needle reverses its direction and begins to travel upward, friction between the thread and the fabric leaves a loop of thread extending below the fabric. This loop of thread is picked up by a shuttle or hook and looped around the bobbin. During this process, the take-up arm moves downward releasing the tension on the thread to allow the loop protruding from the bottom of the fabric to be passed around the bobbin. After the loop has passed around the bobbin effectively crossing the needle and bobbin threads below the fabric, the take-up arm again moves up drawing the slack from the loop around the bobbin and tightening the stitch. The check spring disposed between the tension disc assembly and the take-up arm serves to cushion any increase in thread tension that may be caused by the action of the take-up arm during this stitch tightening process. The check spring additionally functions to determine the time at which pressure on the upper thread is released prior to loop formation. An increase or decrease in the preload of the check spring reduces or increases the amount of thread available for loop formation and delays or advances, respectively, the timing of loop formation.
Problems with prior art tension control systems of the infinitely variable type result from variations in frictional forces generated between the thread and the fabric, or variations in pressure along the stitch line. Characteristically, these variations occur when sewing with different types and thicknesses of thread and fabric and may also occur with changes in denier of any given sample of thread. These variations cause uneven amounts of thread to be available for the formation of any given stitch leading to uneven stitch formation. For example, when sewing over seams or overlapped fabrics, changes in pressure along the stitch line cause the take-up arm to pull too much additional thread off the spool. As a result, the take-up arm fails to pull the stitches up as tightly and uniformly as they should be pulled.
Prior art infinitely variable thread tension control devices apply comparatively large amounts of pressure to the thread as it is being pulled between the tension discs. This can cause wear or grooving of parts of the tension disc assembly that are vital to its operation. This wear problem is accelerated by the use of modern threads such as polyester which were not in general use at the time that many of these prior art tension control devices were designed.
Another problem with prior art tension control systems of the type employing a tension disc assembly alone is that the tension disc often can not be disposed as close as possible to the check spring thereby minimizing thread stretching. Again, this is particularly important with modern synthetic threads that easily stretch. Since the coefficient of static friction is higher than the coefficient of dynamic friction energy stored in a stretched length of thread in the tension control system can pull unwanted thread from the spool during portions of the sewing process when the thread first begins moving.
The prior art also reveals examples of sewing machine thread tension control systems including the combination of an infinitely variable tension control device and a tortuous path tension control device. However, these prior art sewing machine thread tension control systems do not solve the aforementioned problems, in particular, these systems do not teach an increase in the range of thread tension available from an infinitely variable thread tension control device or the buffering of components in the thread tension control system from changes in pressure along the stitch line. These prior art systems fail to achieve these goals because of the type of tortuous path tension control devices used and because of the manner in which tortuous path tension control devices are combined in the path of thread travel with infinitely variable type thread tension control devices. The stated goal of these prior art combinations is to create a tension control system comprised of a number of tension control elements having an additive effect on thread tension.